"Paul Kenny, the leader of the GMB engineering union, said he was willing to go to prison to fight any attempt to tighten union laws.

"The Daily Telegraph disclosed last week that ministers were considering forcing public sector unions to provide a minimum level of service of between 20 per cent and 40 per cent during strikes.

"Mr Kenny called for union members to be prepared to break the law if it meant preserving union rights in the workplace. Speaking at the TUC's annual conference in London, he said: If going to prison is the price to pay for standing up to bad laws, then so be it.

"As long as I don't have to share a cell with some expenses-fiddling MP then I am willing to put my hat in the ring."